Interlock circuit

ABSTRACT

An interlock circuit providing electric power to a transformer or other electric device in response to one or more sensed conditions interrupts such power upon the cessation of one of the sensed conditions. The circuit includes at least one interlock switch, a current limiting fuse, a ballast resistor for drawing current through the fuse upon the cessation of one of the conditions to open the circuit, and a relay monitoring operability of the ballast resistor when energized to close an associated switch connecting the circuit for operation.

United States Patent m1 Lamb [451 July 17,1973

[ INTERLOCK CIRCUIT [75] Inventor: John T. Lamb, Mansfield, Ohio [73] Assignee: The Tnppnn Company, Mansfield,

Ohio

[22] Filed: Aug. 31, 1972 [21] Appl. No.: 285,466

[52] US. Cl. 317/136, 200/50 A, 317/16 [51] Int. Cl. H0lh 47/22 [58] Field of Search 317/136, 154, 16;

[56] References Cited UNITED STATES PATENTS 10/1968 Greenberg 219/l0.55 5/1968 Assow et a1 317/16 Primary Examiner-J. D. Miller Assistant Examiner-Harry E. Moose, Jr. Attorney-Edwin E. Donnelly, Jr.

[ ABSTRACT An interlock circuit providing electric power to a transformer or other electric device in response to one or more sensed conditions interrupts such power upon the cessation of one of the sensed conditions. The circuit includes at least one interlock switch, a current limiting fuse, a ballast resistor for drawing current through the fuse upon the cessation of one of the conditions to open the circuit, and a relay monitoring operability of the ballast resistor when energized to close an associated switch connecting the circuit for operation.

10 Claims, 1 Drawing Figure BACKGROUND OF THE INVENTION The invention relates to interlock circuits, and more particularly to an interlock circuit for opening a power circuit in response to the cessation of a sensed condi-. tion.

Prior art interlock circuits for supplying electric power to an electrical apparatus have used fuses or other similar current limiting and circuit opening de-. vices to limit power supplied to the electrical appara: tus, and such prior art circuits performed adequately as long as all elements thereof were operable; however, an undetected failure of a circuit element would result in the loss of the protection the interlock circuit should provide. Although some circuits have used two inter-. lock switches for safety, these circuits would also lose their alleged safe characteristics upon the undetected failure of one of the switches.

SUMMARY OF THE INVENTION Accordingly, a primary object of the invention is to provide an improved,'safe interlock circuit for supply ing power to an electrical apparatus.

A further object of the invention is to provide an im@ proved interlock circuit including an electric power limiting device.

An additional object of the invention is to provide an improved interlock circuit which provides self monitoring to determine operability of the circuit elements.

Another object of the invention is to provide an improved interlock circuit with primary and secondary interlock elements and which monitors the operability of at least one ofthe interlock elements.

A still further object of the invention is to provide an improved interlock circuit including a current limiting device and a resistor for drawing current therethrough, a relay connected to the resistor, and a switch operated in response to the relay.

Still an additional object of the invention is to provide an improved interlock circuit which interrupts electric power supplied to an electric apparatus by the circuit upon occurrence of an unsafe condition.

Still another object of the invention is to provide an improved interlock circuit which interrupts electric power supplied therethrough to a microwave oven when the main power switch fails closed and the oven door is being opened.

These and other objects of the instant invention will become apparent as the following description proceeds.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a schematic circuit diagram of the interlockcircuit of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT nected by a conductor 7 to the first fixed contact 8 of a doublethrow switch 9, and the switchblade 10. thereof is connected to. a ballast resistor 11 which is coupled by a conductor 12, to the second circuit input terminal 3, the switch 9 forming the second interlock switch of the circuit and the switch together with the ballast resistor forming an interlock element 13 of the circuit. The fuse 6v is also connected in series with first and second control switches 14 and 15, for example timer operated and temperature responsive switches or the like;- a secondary relay operated switch 16; and an energy transfer device, such as a transformer 17 which includes a primary coil 18 coupled in the interlock circuit l and a secondary coil 19 magnetically coupled to the primary coil for connection to supply power to an electric device, such as a magnetron, not shown. The transformerprimary coil 18 is also connected by the conductors 20 and 12 to the second input terminal 3 of the interlock circuit.

A second fixed contact 21 of the double throw switch 9- is connected by a conductor 22 to a relay 23, which controls operation of the relay operated switch 16. The relay 23 is also connected by a conductor 24 to a point 25 between the relay operated switch 16 and the transformer 17, and a current limiting resistor 26 and a start switch 27' are connected in parallel with the relay operated switch. In an alternate embodiment the switch 9 may be a single throw switch including the first fixed contact 8 and the switchblade 10 being adjustable between an open position and a closed position connecting to the first fixed contact. In such alternate embodiment the second fixed contact 21 of the switch 9 is replaced by a fixed connection 28 between the conductor 22 and the ballast resistor 11.

The operation of the interlock circuit 1 will be described hereinafter with reference to use as a power supply through a transformer connection for a magnetron in a microwave oven wherein the oven door must be closed and latched for safety to prevent radiation leakage during operation. The closed door, the first condition, is sensed by the second interlock switch, and the latched door, the second condition, is sensed by the first interlock switch. However, the interlock circuit may be used with any other device requiring a power supply operable in response to the occurrence or the cessation of one or more conditions as sensed by one or more respective interlock switches, as will become more apparent as the description proceeds.

In operation the main circuit switch 4 is manually closed, and the first and second control switches 14 and 15 are properly adjusted, for example by setting a timer or thermostat to close same. When the microwave oven door is closed, occurrence of the first condition. the switchblade 10 of the second interlock switch 9 is thrown away from the first fixed contact 8 to connect with the second fixed contact 21 thereof, coupling the relay 23 through the ballast resistor 11 to the conductor l2 and the second circuit input terminal 3. In the alternate embodiment using a single throw interlock switch, upon occurrence of the first condition, the switchblade 10 is thrown to the open position, the circuit for relay 23 already being made through the fixed connection 28 to the ballast resistor 11. As the second condition occurs, for example the latching of the microwave oven door, the first interlock switch 5 is closed thereby providing a potential energization circuit to the 3 transformer 17 but for the open relay operated switch 16.

The start switch 27 is closed permitting current to flow through the current limiting resistor 26 and the start switch to energize the relay 23 which closes the relay operated switch 16 providing a closed circuit to the transformer 17, after which the start switch may be released, and the relay will remain energized by current flowing from the point 25 in the circuit through the conductor 24, relay 23, and ballast resistor 11, and to the second input terminal 3 of the interlock circuit. If the ballast resistor 11 were defective or otherwise inoperable, creating an open circuit thereacross, then current flow through the relay 23 would be interrupted, and the relay operated switch 16 would open, interrupting current flow to the transformer 17. The relay 23 thus forms a continuous monitoring circuit to determine operability of the ballast resistor 11, and in the embodiment using the double throw switch 9, the operability thereof is also monitored.

The current flow to the transformer 17 may be interrupted by the opening of any one of the main circuit switch 4, first interlock switch 5, first and second control switches 14 and 15, or the relay operated switch 16. The latter three switches are normally operated automatically, and, thus, for manual operation the current flow in the circuit is generally interrupted by opening the main circuit switch 4. If the main circuit switch 4 were to fail in closed position, for example by welding due to arcing created by high electric currents therein, then the first interlock switch 5 would open the circuit to the transformer 17 when the microwave oven door is unlatched eliminating one of the above conditions. However, if a similar high current arcing would also cause the first interlock switch 5 to weld in closed position, thus still permitting current flow to the transformer 17, the interlock element 13, including the second interlock switch 9 and the ballast resistor 11, would then be operable to draw destructive current through the fuse 6 upon opening the oven door.

As the microwave oven door is opened, eliminating one of the above conditions, the switchblade 10 of the second interlock switch 9 is thrown to abut the fixed contact 8 thereof, forming with the ballast resistor 11 a parallel circuit across the first and second control switches 14 and 15, the relay operated switch 16, and the transformer 17, the ballast resistor drawing a high current through the fuse 6, which current causes same to open the circuit therethrough interrupting current flow to the transformer.

In normal circuit operation since current does not flow through the single throw interlock switch and only a small current from the relay 23 flows through the double throw interlock switch and since the second interlock switch 9 in either the single or double throw embodiment is only switched when the circuit 1 is not energized, it is unlikely that it would fail. Also, since the ballast resistor 11 always must be operable to draw current through the fuse 6 in the event of a failure in the main circuit switch 4 and the first interlock switch 5, to insure that the fuse will open the circuit to the transformer 17, the relay 23 is connected to monitor such operability. Specifically, if the ballast resistor 11 were to fail causing an open circuit, current through the relay 23 would be interrupted allowing the relay operated switch 16 to open thereby de-energizing the transformer 17.

As can now be seen from the foregoing description, the invention provides an improved interlock circuit which includes numerous safety features and fail safe devices to stop current flow to an energy transfer device when an emergency caused by the failure of one or more circuit elements occurs.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A circuit for supplying energy to an electric apparatus comprising main switch means for coupling the circuit to a source of electric energy, means for opening the circuit in response to excessive electric energy in same, means for coupling energy in the circuit to the electric apparatus, means for drawing excessive electric energy through said main switch means and said means for opening when a failure occurs in the former to effect opening of the latter, secondary switch means for electrically connecting said main switch means and said means for opening to said means for coupling, and means responsive to operability of said means for drawing for operating said secondary switch means, whereby upon sensing inoperability of said means for drawing said means for operating is disabled from operating said secondary switch means.

2. A circuit as set forth in claim 1 wherein said means for opening comprises fuse means.

3. A circuit as set forth in claim 1 wherein said means for coupling comprises transformer means.

4. A circuit as set forth in claim 1 wherein said means for drawing comprises resistor means.

5. A circuit as set forth in claim 4 wherein said means for drawing comprises further switch means.

6. A circuit as set forth in claim 5 wherein said further switch means comprises a double throw switch having a first position to permit the drawing of current through said means for opening upon failure of said main switch means and a second position to permit energization 'of said means for operating when said resistor means is operable.

7. A circuit as set forth in claim 1 wherein said secondary switch means comprises a relay operated switch means.

8. A circuit as set forth in claim 7 wherein said means for operating comprises a relay.

9. A circuit as set forth in claim 1 wherein said means for operating comprises a relay.

10. A circuit as set forth in claim 1 wherein said main switch means, said means for opening and said secondary switch means are connected in series, and said means for drawing is connected in parallel with said means for coupling.

I. t I! l 

1. A circuit for supplying energy to an electric apparatus comprising main switch means for coupling the circuit to a source of electric energy, means for opening the circuit in response to excessive electric energy in same, means for coupling energy in the circuit to the electric apparatus, means for drawing excessive electric energy through said main switch means and said means for opening when a failure occurs in the former to effect opening of the latter, secondary switch means for electrically connecting said main switch means and said means for opening to said means for coupling, and means resPonsive to operability of said means for drawing for operating said secondary switch means, whereby upon sensing inoperability of said means for drawing said means for operating is disabled from operating said secondary switch means.
 2. A circuit as set forth in claim 1 wherein said means for opening comprises fuse means.
 3. A circuit as set forth in claim 1 wherein said means for coupling comprises transformer means.
 4. A circuit as set forth in claim 1 wherein said means for drawing comprises resistor means.
 5. A circuit as set forth in claim 4 wherein said means for drawing comprises further switch means.
 6. A circuit as set forth in claim 5 wherein said further switch means comprises a double throw switch having a first position to permit the drawing of current through said means for opening upon failure of said main switch means and a second position to permit energization of said means for operating when said resistor means is operable.
 7. A circuit as set forth in claim 1 wherein said secondary switch means comprises a relay operated switch means.
 8. A circuit as set forth in claim 7 wherein said means for operating comprises a relay.
 9. A circuit as set forth in claim 1 wherein said means for operating comprises a relay.
 10. A circuit as set forth in claim 1 wherein said main switch means, said means for opening and said secondary switch means are connected in series, and said means for drawing is connected in parallel with said means for coupling. 